The present invention relates generally to laser light sources and more particularly to a second harmonic generator which can stably generate a second harmonic laser light from a fundamental wave laser light.
A laser light source has been proposed, which can emit a laser light with a short wavelength, by producing a second harmonic laser light having a frequency twice as high as the frequency of a fundamental wave laser light, produced within a resonator of a laser light source (see Official Gazette of Laid-Open Japanese Utility Model Application No. 48-937845).
This kind of laser light source can efficiently emit a second harmonic laser light by phase-matching the second harmonic laser light with the fundamental wave laser light, in a non-linear optical crystal element provided within the resonator containing a laser medium.
For realizing the phase-matching, a phase matching condition of type I or type II has to be established between the fundamental wave laser light and the second harmonic laser light.
The phase-matching condition of type I is based on the principle that, by utilizing an ordinary ray of the fundamental wave laser light, one photon having a frequency twice as high as the fundamental is produced from two photons polarized in the same direction, as expressed in the following equation ##EQU1## If the fundamental wave laser light is polarized such that its polarized direction is made coincident with the direction of the non-linear optical crystal element by utilizing a polarizing element such as a polarizing-type beam splitter or the like so as to become incident, in principle, polarized-components (p-wave component and s-wave component) of the fundamental wave laser light emitted from the non-linear optical crystal element can be prevented from being changed in phase, thus making it possible to stably and continuously emit the second harmonic laser light on the basis of the fundamental wave laser light resonating within the resonator. The above-mentioned p-wave component and s-wave component are referred to as intrinsic polarization components.
On the other hand, to realize the phase-matching condition of type II, the phase-matching condition must be established, respectively for two intrinsic polarization components, by introducing two fundamental wave intrinsic polarized lights, which are perpendicular to each other, into a non-linear optical crystal element. That is, the fundamental wave laser light is divided into an ordinary ray and an extraordinary ray within the non-linear optical crystal element, thereby to be phase-matched with the extraordinary ray of the second harmonic laser light as is expressed in the following equation (2) ##EQU2## In the equations (1) and (2), n.sub.o(w) and n.sub.e(w) respectively depict refractive indexes of the fundamental wave laser light (frequency f=w) relative to the ordinary ray and the extraordinary ray, respectively, while n.sub.o(2w) and n.sub.e(2w) respectively depict refractive indexes of the second harmonic laser light (frequency f-2w) relative to the ordinary ray and the extraordinary ray, respectively.
If the second harmonic laser light is produced by utilizing the phase-matching condition of type II, however, each time the fundamental wave laser light repeatedly passes the non-linear optical crystal element, the phase of the intrinsic polarized light of the fundamental wave laser light is changed, resulting in the risk that the second harmonic laser light cannot be generated stably and continuously.
Specifically, if the phases of the perpendicular intrinsic polarized lights (namely, the p-wave component and the s-wave component) are displaced respectively each time the fundamental wave laser light generated by the laser medium repeatedly passes the non-linear optical crystal element by the resonating operation, a stationary state in which the fundamental wave laser lights each increase their light intensities at the respective portions of the resonator efficiently cannot be obtained, thus making it impossible to establish a strong resonation state (namely, a strong standing wave). As a result, the efficiency in which the fundamental wave laser light is converted into the second harmonic laser light is lowered. Also, there is a risk that noise will occur in the second harmonic laser light.